Tension, voltage, electromotive force or difference of electrical potential is the unit to measure work performed by one electrical charge of 1 Coulomb while moving in a uniform electrical field. This definition may sound a little complex, right? In order to make it easier, imagine an energy outlet in your house. Between both pins there is a tension of 127 or 220 Volts, depending on the carrier. The tension between 2 pins will allow that any electrical device plugged into it works correctly, according to its functions.
In reality, what happens when we plug an electric device into an energized outlet (having tension between 2 pins) an electric current will circulate in the circuit and the current will produce desired effect on the device (if it is a lamp, it will light; if it is a motor it will start; if it is a heater, it will produce heat).
In a battery the same thought applies, with the difference that in the outlet, the difference of potential is between the pins while in the battery it is between the poles. Another basic difference it is that the energy supplied by the carrier is the result of continuous action of generators while in an automotive battery there is not a generator, but an accumulator of energy.